1. Field of Invention
The present invention relates to utilization of natural cold ambient air and mechanical refrigeration, and more particularly to method and system for transmitting and storing cold medium during cold seasons, preferably underground, for effective uses during summer seasons.
2. Description of the Prior Art
Prior art ground cold storage is generally accomplished by blowing winter cold ambient air through a cold reservoir formed by a bed of crushed stones and rocks. In other cases, cold liquid medium is pumped through pipes embedded in stone and rock medium in the form of a closed heat-exchange system.
The stones and rocks, as a cold storage reservoir, have several shortcomings. First, cold storage capacities are limited because of the narrow temperature range for cooling. In other words, the difference of storage temperature in winter and upper-limit useful temperatures in summer is less than thirty degrees Fahrenheit (30.degree. F. to 60.degree. F.). For example, in a volume of one cubic foot of stone at a temperature difference of 30.degree. F., the energy flux is about 1.3.times.10.sup.3 BTU. Assuming a summer requirement of 25.times.10.sup.6 BTU for an average size home, a volume of cold storage required for cooling is estimated at 20.times.10.sup.3 cubic feet. In contrast to this large volume, if ice is used for cold storage instead of rocks, the required storage volume is only about 2.5.times.10.sup.3 cubic feet. Ice changes from liquid to solid, or vice versa, at 32.degree. F. with liberation or absorption, respectively, at 143 BTU per pound of ice.
Another shortcoming of the stones and rocks as cold storage is the low thermal conductivity. It would require a long length of liquid pipes considerable pumping power in order to meet the maximum cooling rate required during hot summer days.